I was a smart kid. The only problem I had related to believing things was adults not believing me when I said I could count to 60 during a discussion of minutes.
It's not that abnormal for a two-year-old to be able to count into the hundreds along with basic addition and multiplication. (I was always bad with division though, all the way up to 2nd grade. I was more proficient with exponents and basic algebra than I was with division. Not sure why.)

Of course, I hit the nihilistic stage pretty early, so as far as Santa goes... it went straight from "Who is Santa?" to "Pfft, it's totally illogical for that to happen."

Damn, this topic made me feel weird. But it was just my childhood, so it's fine I suppose.